The invention relates to a circuit for operating a load such as, for example, a low-pressure discharge lamp.
At issue, in particular, is an operating circuit for power ranges of up to a few tens of Watts, in which centre stage is occupied first and foremost by costs, for example small power supply units and, in particular, also electronic ballasts (EB) for compact fluorescent lamps (KLL).
In such circuits, the switching elements have to be supplied with a driving power in step with the operating frequency. For this purpose, the known arrangements make use of separate current transformers, as shown in the book entitled "Elektronikschaltungen" ("Electronic circuits") by W. Hirschmann (Siemens AG, 1982) on pages 148 and 150, for example, or, via booster windings on the resonance reactor or the power transformer, they tap control energy which they reshape appropriately in a pulse shaping and phase shifter network (see German Offenlegung DE 41 29 430). It is also possible, of course, to use frequency or pulse width control, preferably integrated in a control IC.
However, a disadvantage in such controls is the outlay in the form of dedicated winding materials or additional windings on existing inductors, or of complicated and expensive control ICs with an auxiliary power supply etc.; said outlay greatly influences the costs precisely in the case of low powers since, by contrast with the power elements, the cost of control is virtually exactly as high as for a few hundred Watts even in the case of a transmitted power of only a few Watts.
The invention is based on the technical problem of improving the circuit specified at the beginning with regard to simpler and more cost-effective production.